Altered GABAA Receptor Density and Unaltered Blood Brain Barrier Transport in a Kainate Model of Epilepsy: An In Vivo Study Using 11C-Flumazenil and PET

The aim of the present study was to investigate if flumazenil blood-brain barrier transport and binding to the benzodiazepine site on the gamma-aminobutyric acid A (GABA(A)) receptor complex is altered in an experimental model of epilepsy and subsequently to study if changes in P-glycoprotein (P-gp)-mediated efflux of flumazenil at the blood-brain barrier may confound interpretation of C-11-flumazenil PET in epilepsy. Methods: The transport of flumazenil across the blood-brain barrier and the binding to the benzodiazepine site on the GABA(A) receptors in 5 different brain regions was studied and compared between controls and kainate-treated rats, a model of temporal lobe epilepsy, with and without tariquidar pretreatment. In total, 29 rats underwent 2 consecutive C-11-flumazenil PET scans, each one lasting 30 min. The tracer was mixed with different amounts of isotopically unmodified flumazenil (4, 20, 100, or 400 mu g) to cover a wide range of receptor occupancies during the scan. Before the second scan, the rats were pretreated with a 3 or 15 mg/kg dose of the P-gp inhibitor tariquidar. The second scan was then obtained according to the same protocol as the first scan. Results: GABA(A) receptor density, B-max, was estimated as 44 +/- 2 ng.mL(-1) in the hippocampus and as 33 +/- 2 ng.mL(-1) in the cerebellum, with intermediate values in the occipital cortex, parietal cortex, and caudate putamen. B-max was decreased by 12% in kainate-treated rats, compared with controls. The radiotracer equilibrium dissociation constant, K-D, was similar in both rat groups and all brain regions and was estimated as 5.9 +/- 0.9 ng.mL(-1). There was no difference in flumazenil transport across the blood-brain barrier between control and kainate-treated rats, and the effect of tariquidar treatment was similar in both rat groups. Tariquidar treatment also decreased flumazenil transport out of the brain by 73%, increased the volume of distribution in the brain by 24%, and did not influence B-max or K-D, compared with baseline. Conclusion: B-max was decreased in kainate-treated rats, compared with controls, but no alteration in the blood-brain barrier transport of flumazenil was observed. P-gp inhibition by tariquidar treatment increased brain concentrations of flumazenil in both groups, but B-max estimates were not influenced, suggesting that C-11-flumazenil scanning is not confounded by alterations in P-gp function.